Update Go versions used in travis and LANGUAGES.md

Per our support policy, drop support of go 1.16 and add support of go
1.18.

Also enable go vet in make check, and fix issues reported by go vet.
diff --git a/test/go/genmock.sh b/test/go/genmock.sh
index 4958783..9bd2a3c 100644
--- a/test/go/genmock.sh
+++ b/test/go/genmock.sh
@@ -4,7 +4,7 @@
 
 export GOPATH=$(mktemp -d -t gopath-XXXXXXXXXX)
 
-GO111MODULE=on go install -mod=mod github.com/golang/mock/mockgen
+go install -mod=mod github.com/golang/mock/mockgen
 
 `go env GOPATH`/bin/mockgen -build_flags "-mod=mod" -destination=src/common/mock_handler.go -package=common github.com/apache/thrift/test/go/src/gen/thrifttest ThriftTest